Multiple motor control system



Oct 1963 'r. CHENG ET AL MULTIPLE MOTOR CONTROL SYSTEM Filed June 14, 1961 United States Patent 3,ltl9,13tl MULTKELE MGTGR CGNTRGL SYSTEh i Theodore Cheng, Wynnewood, Warren D. ll/iael labh, Oreland, and Harold A. List, Ambler, assignors to Leeds and Northrup Company, Philadel hia, Pa, a

corporation of Pennsylvania Filed June 14, 196i, Ser. No. ll'l,ltl9 Claims. (C1, SIS-72) This invention relates to control systems of the type applicable to the control of a plurality of units which are arranged to cooperate one with another to produce a combined output to meet a load demand on the system and at the same time to maintain a predetermined sharing of that demand between individual units.

As explained in Bristol Patent 2,657,347, dated October 27, 1953, in a modern boiler or vapor generator a plurality of burners are utilized. The total needed fuel supply must be divided among the several burners in a manner not only to secure desired combustion conditions, but also to achieve a desired distribution of heat within the furnace. In some instances it may be desired to increase the rate of fuel supply to the upper set of the burners relative to the lower set of burners to increase the temperature of the combustion gases flowing through a superheater section, and in other cases it may be desired relatively to increase the rate of the fuel supply to the lower burners. In any case, it is necessary that the total fuel supplied to the combustion zone of the steam generator shall meet the demand on the generator, that demand in general being determined by the load upon associated turbine driven generators or alternators.

While the system of said Bristol patent provides one solution to the problem, something has been left to be desired in the achievement of the same end results in a dififerent way. Thus, in the Bristol patent the control of the individual final control elements is required to follow one of the final control elements which serves as a master unit, and whenever an individual final control element gets out of step with the master, it is immediately forced back into step, i.e., made to maintain a predetermined relationship relative to the other final control elements. A control system of the foregoing type has sometimes been referred to as an active control circuit.

In accordance with the present invention, all final control elements are jointly under the control of a master controller, and the outputs rise and fall together so long as they are in step. When one final control element ge out of step by reason of a smaller or greater response than the others, the effect of the master controller on that final control element is discontinued in that direction until the remaining final control elements are again in step with it. This type of control has been referred to as permissive. To achieve permissive control, there is utilized a parallel permissive circuit equalization network, each branch of which includes a signal source proportional to the output of a final control element, a biaaproducing means for developing an output voltage proportional to the desired relative output of the final control element relative to the outputs of the remaining final control elements, together with a detector. The latter is utilized to control the application of change-ofoutput signals from the master controller to the respective final control elements and to prevent change in output of a final control element in direction to increase the deviation of the output from a desired magnitude.

For further objects and advantages or the invention and for a more detailed discussion thereof, reference is to be had to the following description, taken in conjunction with the accompanying drawing.

Referring now to the drawing, the invention in one form has been shown as applied to a plurality of final control elements 1ll13, each comprising feeding devices for supplying coal to pulverizing devices 14-47. These devices include a fan for transporting powdered or pulverized coal through lines lit-21 to a plurality of burners 22a-25a and their counterparts 2211-4512 which receive pulverized coal from the same supply lines lid-21 as indicated by the branch lines l8a21a, only the latter line 21a being shown connected to the lowermost burner 25b. It will be understood that the rate of supply of fuel to each burner will be dependent upon the speed of operation of the respective feeding devices lit-13. These devices are driven by electric motors 3tl33, each motor having speed-adjusting means associated therewith and shown in the form of series resistors 34-37. The motors 3e33 respectively drive tachometer generators ill -43, the four of them being connected in a series circuit including a coil 44 of a balanceable master controller 45. The master controller 45 includes a centrally pivoted arm 46 which in response to the relative degrees of energization of coil 44 and a second coil 47 operates the movable blade of a single-pole doublethrow switch 43 into and out of engagement with two stationary contacts. The upper contact is connected to a conductor 49 for application to actuating means for the final control elements of lowering signals, and the other lower contact being connected to a conductor Sil for application to the actuating means of raise signals.

The coil 47 located on the opposite side of the pivotal axis of the arm 46 from the coil 44 is energized by any suitable means 51 in response to the fuel requirements of the burners 22a25a or 2212-4515. As shown, these burners are associated with a furnace 52 which may be a steam generator of any suitable type, such for example, as shown in said Bristol Patent 2,657,347. The condition-responsive device 51 may either apply to the pivoted arm as a mechanical force as shown in said Bristol patent, or that force may be that developed from the coil 47 and of magnitude dependent upon the extent of its energization. More particularly, the coil 4'7 may be energized by the voltage E of FIGS. 1 and ID of a llriston Patent 2,861,194. For the purpose of the present invention, the energization of the coil 47 will be proportional to the fuel demand of the furnace or boiler 52, and the energization of coil 44 will be proportional to the sum of the voltages developed by the tachometers If the fuel demand exceeds the rate of :fuel delivery to the plurality of burners, the coil 4'7 will rotate pivoted arm 46 in a clockwise direction to close a circuit between the movable blade of switch 43 and the lowermost contact to complete a circuit by way of the conductor 5%. With this circuit closed, there will then be periodically applied to the conductor raise pulses developed by the periodic closure of a pulse-generating switch 53 which is opened and closed by means of a cam 54 driven by a motor 55. With the parts in the illustrated positions, but with the aforesaid circuit through switch 48 closed, it will be seen that current may how by way of the positive terminal 57 of a suitable source or" supply, switch contacts 53, the switch 43, and by way of conductor to the actuating means for the final control elements Iii-13. As shown, the means include the raise windings 666 of motors 6265, each said motor having mechani al driving connections, as indicated by the broken lines, respectively extending to the adjustable contacts of the speed-varying devices 34-37. Each motor circuit is completed by way of the armature and thence to the negative terminal 7t? of the source of supply.

It will be noted that each of the aforesaid motor circuits is completed by Way of the contacts of relays 71-74. Had the demand for "fuel been less than the rate of fuel supply, the force developed by the coil 44 would have exceeded that developed by the coil 47, and a lowering pulse would then be applied by way of conductor 49 to the respective motors 6265 by way of the lower windings 7679, these circuits again being completed respectively by way of the contacts of relays 81-84.

In the above description the concurrent energization of the motors d265 for rotation in one direction or the other takes place so long as each of the feeding devices or final control elements llfil3 is supplying its desired share of the load. For the moment, it may be assumed that the final control elements are equally to divide the load requirements. That division will be achieved unless one of motors 3tl33 produces a speed of ope-ration of one of the fedding devices lltll3 difiering from the remaining feeding devices. This means that the final control elements 1ll'-13 will not remain in step since one of them may tend to assume more or less than its share of the load.

To assure that there will be desired sharing of the load among the final control elements lit-13, the relays 7l74 and 3Zl8 i have been provided in association with a relay control circuit of the parallel type including conductors 91 and 92 and provided with a plurality of branch circuits 93-96. The branch circuits respectively include identical phase-sensitive relay amplifiers 97-109, only one of which, the amplifier 97, has been shown in detail. Each of the branch circuits also includes biasing means shown as load-determining slidewires 191-164, and each branch circuit also includes one of the follow-up slidewires lil5i"8. The movable contacts of the slidewires li5-lfi8 are respectively operated by the mechanical connect-ions (shown by broken lines) to the respective motors 62-d5. The slidewires of each branch circuit are energized from one-half of a center-tapped secondary Winding of transformers TIL-414. As shown, the movable contacts of the slidewires till-J04 have been illustrated in their mid-positions. Similarly, the follow-up contacts associated with slidewires ltl51il8 have been shown in their mid-positions. As a result, the net voltage in each branch circuit, as developed by the slidewires therein, is equal to the voltage similarly developed in each or" the other branches.

The input circuit to the amplifier 97 extends between the movable contact of slidewire 165 and the conductor 91. This input voltage with the parts in the positions illustrated will be zero for the reason that instantaneous values of the voltage developed between the contacts of slidewires lili and 105 in the first branch circuit 93 will be equal and opposite to the voltages developed in the remaining branch circuits. In the input circuit to amplifier 97 the net voltage will then be zero.

The foregoing condition of balance will be altered whenever the motor 62 adjusts the speed-varying means 34 of motor Ed by a finite amount differing from the adjustments made by the remaining motors 63-55 of their corresponding speed-varying means 35-37.

Assuming now that the motor 62 has eifected a greater adjustment than the remaining motors and in direction to cause a greater amount of fuel to be delivered by the feeding device than the remaining devices ll113, then there will be applied to the primary winding of the input transformer 11b of amplifier 9'7 an input signal of phase which at the secondary winding of transformer lit? will, with respect to a phase-reference signal introduced by transformer 1T6, cause a tube 117 to conduct. In this manner, the operating coil of relay '71 will be energized to open its contacts and to interrupt application to the raise winding es of motor of raise pulses. These raise pulses will continue to be applied to the raise windings 67-69 so long as the motors 63-65 move in unison or stay in step. Whenever one of them gets out of step in the above-assumed direction, then the corresponding one of relays '727 will be energized by its corresponding one of amplifiers 98-1ilil to interrupt flow of raise pulses thereto. it is in this manner that with raise pulses delivered to the conductor till, the demandadjusting devices including the motors 63-65 are brought back into step with the demand-adjusting device shown as motor 62.

Continuing with the foregoing assumptions, and particularly that the motor 62 is out of step, and now making the further assumption that the demand has changed from a requirement of raise pulses to one of lower pulses, it will be seen at once that the relay 31 has its contacts closed and, therefore, a lower pulse will be applied immediately to the lower winding '76 toreverse the direction of rotation of motor 62 and at the same time to readjust the contact of slidewire M5.

The lower pulses are not correspondingly applied to the windings TI -79 of motors 6365 for the reason that in the above description the unbalance resulting in the input voltage to the input transformer 11d applies to each of the input circuits to amplifiers hS-ltlti input signals of opposite phase. Each amplifier thereupon energizes the operating coils of relays 8284 to prevent application to motors of the lowering pulses. In this manner, the control system quickly returns the motors 6265 to their desired operation in maintaining predetermined division of the load requirements among the final control elements l;l3.

The foregoing operation of each of amplifiers 931t29 will be readily understood by assuming that the input signal to the transformer 11d of amplifier $7 is now of opposite phase to the input signal previously described. Such an input signal will cause the tube 118 to be conductive to energize the relay 31, the output circuit being readily traced from the secondary winding of the reference transformer 116 which is also the plate-supply transformer.

With the above understanding of the invention, it will now be seen that each of the knobs associated with the movable contacts of slidewires Mil-104 may be operated to change the relative sharing of the load demand of the several final control elements 16-43. More particularly, it will be assumed that the movable contact of slidewire MM is moved in a counterclockwise direction by a substantial amount. Such manual adjustment correspondingly increases the magnitude of the voltage introduced into the branch circuit 93 by the slidewire 101. Thus the net voltage introduced into branch circuit 93 then exceeds the voltage introduced into the branch circuit 94 by the slidewires It); and 106.

Had the adjustment of the contact of slidewire Iii-1 been in a clockwise direction from its illustrated position, it will be seen that the voltage introduced into the branch circuit 93 would have been made less than the voltage introduced by the slidewires 132 and 106 of branch circuit 94.

The foregoing explanation makes clear the fact that, considering branches 93 and 94- only, the instantaneous directions of current applied to amplifiers 97 and 98 will chan e in direction (phase) as the net voltage of branch 93 exceeds or is less than the voltage introduced into the branch circuit 94. Thus if the voltage of branch 93 is greater, the instantaneous current may be assumed to flow in a clockwise direction around a closed loop including branches 93 and 94. With the voltage of branch 93 less, the corresponding instantaneous current will then be in an opposite direction. Thus the current as applied to amplifiers 9'7 and will in the two cases be of opposite direction, instantaneously considered and of opposite phase from the alternating current standpoint.

Returning again to the assumption that the contact of slidewire lhll occupies the position to the right of its illustrated position, it will then be seen that the unbalance voltage will have an instantaneous direction or phase such as to require the movable contact of slide- Wire 105 to be moved in a counterclockwise direction by motor 62 to restore equality between the branch voltages. This will result in encrgization of the relay 71 to prevent application to the motor as of raise pulses and to permit application thereto of lower pulses to move the contact of slidewire 165 in the required counterclockwise direction and simultaneously energize relay 82 for application of raise pulses only to the motor 63 to move the contact 1&6 in a clockwise direction until the voltage introduced in branch 93 again equals the voltage of branch 94. In this manner, there will be achieved a difference in the setting of the load-adjusting device 36) from that of the device 31 and the remaining devices 32 and 33. The motor 36 then operates at a lower speed than the motors or devices 3133 driving the final control elements 11-13, and thus reduces the heat developed by the uppermost burners 22a, 22b relative to the lower burners.

It may be further noted that with the contact of slidewire 101 in its above-described position to the right of that shown in the drawing, a change in load demand as from the master controller 45 will result in equal changes in the delivery of fuel to the burners by the devices ltl 13. However, the change in rate of delivery of fuel by the device will be from a lower rate than will be the change which takes place at the remaining final control elements 11-13.

From the above description, it will be seen that the several final control elements Ill-13 may, by means of the adjustment of the contacts associated with slidewires lo l- 194, be set for desired rates of flow of fuel to their associated burners and that these several final control elements will be adjusted equally to share in changes of load demand in either direction. However, for a given load, the final control elements Ill-13 will share the load in predetermined division as established by the setting of the contacts of slidewires lilllltl l.

Though the invention has been described in terms of speed of rotation of motors 36-33 and of speed of rotation of the feeder devices comprising final control elements 10-13, it will be understood that the present invention may be utilized in connection with many different kinds of final control elements and, specifically, elements in which position changes the output or modifies the conditions under control. In combustion control, the positioning of throttling elements for oil or gas fuel provides full counterparts of the feeding devices iii- 13 for the pulverized coal used for the above explanation. The tachometer summing circuit including the coil 44 suggests that other types of conventional summing circuits may be utilized with voltages developed therein by slidewires in response to position changes of the final control elements. The amplifiers themselves have been described as phase sensitive. However, the description has been in terms of both instantaneous polarities and phase. Accordingly, it will be seen that direct current amplifiers and direct current circuits may be utilized in accordance with the invention, the scope of which is to be determined by reference to the appended claims.

What is claimed is:

1. A control system for maintaining a desired relation between the outputs of a plurality of final control elements comprising master means responsive to the magnitude of a controlled variable for varying the outputs of said final control elements in directions to maintain said controlled variable at a desired value, output signal producing means for producing output signals representative of the individual outputs of said final control elements, biasing means for each of said final control elements for establishing reference levels representative of relative outputs therefrom, signal detecting means for each of said final control elements, a circuit having a plurality of parallel branches,

one for each of said final control elements and each branch including in series circuit relation for one of said final control elements its said corresponding output signal producing means, biasing means and detecting means, said detecting means of each said branch having its circuit completed through the remaining of said branches for response to the average departure of all final control elements from their reference levels and to the departure of its final control element from its reference level and means intermediate said master means and said final control elements respectively responsive to said detecting means for modifying the action of said master means in varying the outputs of said final control elements iwhenever their said outputs depart from a predetermined relationship.

2. A control system for a plurality of final control elements comprising a control circuit having a plurality of parallel branches, one for each of said final control elements, each said branch including a signal producing means operable in accordance with change of output from a corresponding one of said final control elements, a biasing means, and a detecting means, each said detecting means responding to the difference between the potential difference across said control circuit and the net potential difference developed in its said branch thereof, actuating means for each of said final control elements, output signal producing means for each said final control element for producing an output signal representative or" the output of each said final control element, a master controller having a summing means for application thereto of the sum of said output signals and having a second means for applying thereto an input representative of the sum of the outputs required of said final control elements, control circuits jointly operable under the control of said master controller for varying the outputs of said final control elements, and means operable under the control of said detectors for preventing change in one direction of the output of any control element which has been operated to produce an output differing in the same direction from the outputs of the remaining final control elements.

3. A control system for predetermining and maintaining the relative magnitudes of the outputs of a plurality of final control elements comprising a control circuit having a plurality of parallel branches, one for each of said final control elements, each said branch including output signal producing means for developing therein an output signal representative of the output of its associated final control element, a biasing means for establishing a bias signal representative of the output of its associated final control element relative to the outputs of the remaining final control elements, and a detecting means for its associated final control element, means including signal directing means for controlling the outputs of said final control elements, a master controller responsive to the difference in magnitude between the combined outputs of said final control clemerits and a signal representative of the sum of the outputs required of said final control elements for applying control signals to said direming means, and means operable in response to said detectors for operating said directing means for application of said control signals from said master controller to said final control elements for concurrently changing their outputs and for preventing application thereto of signals for changing the output of any final control element in a direction in which its output already differs fromthe outputs of the remaining final control elements.

4. A control system for maintaining a desired relation between the outputs of a plurality of final control elements comprising master means responsive to the magnitude of a controlled variable for varying the outputs of said final control elements in directions to maintain said controlled variable at a desired value, output signal producing means for producing output signals representative of the individual outputs of said final control elements, biasing means for each of said final control elements for establishing reference levels representative of relative outputs therefrom, signal detecting means for each of said final control elements, a circuit having a plurality of parallel branches, one for each of said final control elements and each branch including for one of said final control elements its said corresponding output signal producing means, its corresponding biasing means and its corresponding detecting means, said detecting means of each said branch having its circuit completed through the remaining of said branches for response to the average departure of all final control elements from their reference levels and -to the departure of its final control element from its reference level and circuit-controlling means intermediate said master means and said final control elements responsive to said detecting means for preventing operation by said master means of one of said final control elements in one direction when the output of that final control element departs from a predetermined relationship with respect to the output of another of the remaining final control elements.

5. A control system for maintaining a desired relation between the outputs of a plurality of final control elements comprising master means responsive to the magnitude of a controlled variable for varying the outputs of said final control elements in directions to maintain said controlled variable at a desired value, output signal producing means for producing output signals representative of the individual outputs of said final control elements, biasing means for each of said final control elements for establishing reference levels representative of relative outputs therefrom, each of said biasing means being individually adjustable for establishing outputs for said final control elements which differ one from the other, signal detecting mean for each of said final control elements, a circuit having a plurality of parallel branches, one for each of sad final control elements and each branch including for one of said final control elements its said correspond ing output signal producing means, its corresponding biasing means and its corresponding detecting means, said detecting means of each said branch having its circuit completed through the remaining of said branches for response to the average departure of all final control eleents from their reference levels and to the departure of its final control element from its reference level and circuit-controlling means intermediate said master means and said final control elements responsive to said detecting means for preventing operation by said master means of one of said control elements in one direction when the output of that final control element departs from a predetermined relationship with respect to the outputs of the remaining final control elements.

References Cited in the file of this patent UNITED STATES PATENTS 2,657,347 Bristol Oct. 27, 1953 FOREEGN PATENTS 534,039 Canada Dec. 4, 1946 

1. A CONTROL SYSTEM FOR MAINTAINING A DESIRED RELATION BETWEEN THE OUTPUT OF A PLURALITY OF FINAL CONTROL ELEMENTS COMPRISING MASTER MEANS RESPONSIVE TO THE MAGNITUDE OF A CONTROLLED VARIABLE FOR VARYING THE OUTPUTS OF SAID FINAL CONTROL ELEMENTS IN DIRECTIONS TO MAINTAIN SAID CONTROLLED VARIABLE AT A DESIRED VALUE, OUTPUT SIGNAL PRODUCING MEANS FOR PRODUCING OUTPUT SIGNALS REPRESENTATIVE OF THE INDIVIDUAL OUTPUTS OF SAID FINAL CONTROL ELEMENTS, BIASING MEANS FOR EACH OF SAID FINAL CONTROL ELEMENTS FOR ESTABLISHING REFERENCE LEVES REPRESENTATIVE OF RELATIVE OUTPUTS THEREFROM, SIGNAL DETECTING MEANS FOR EACH OF SAID FINAL CONTROL ELEMENTS, A CIRCUIT HAVING A PLURALITY OF PARALLEL BRANCHES, ONE FOR EACH OF SAID FINAL CONTROL ELEMENTS AND EACH BRANCH INCLUDING IN SERIES CIRCUIT RELATION FOR ONE OF SAID FINAL 